This invention relates to polyurethane elastomer compositions, and more particularly to polyurethane compositions having shock absorbing properties combined with relatively short recovery times. The materials are also particularly well suited for use in shock absorbing, sound damping and other applications in which absorption of energy applied in cycles of relatively short duration are required. The materials are particularly well suited for use as shock absorbing elements in footwear, such as soles and insoles for walking and running shoes.
In the act of walking, the foot of an average-size man decelerates with a force seven to eight times the normal force of gravity. In running, the deceleration increases to fifteen to twenty times the normal force of gravity. In walking, the deceleration occurs cyclically at about fifteen to forty millisecond intervals and at intervals of from 15 to 25 milliseconds in running. The mechanical implications, i.e., shock waves, associated with heel strike in running and walking are of major concern to medical practitioners.
It is thus seen that there is a need for a shock absorbing material capable of absorbing substantial shocks applied at 15 to 100 millisecond intervals, as are encountered in normal exercise and other similar environments.
Prior art viscoelastic polymers capable of dispersing considerable levels of kinetic energy suffer from two major disdvantages. They are comparatively heavy and their recovery rate is too slow. The ideal properties of an energy absorbing elastomer, and those attained in accordance with the present invention, are:
1. It must deform readily, for in this way it disperses the impacting forces over the maximum available area;
2. On compression, it should become progressively stiffer in bringing to a halt the downward movement of, for example, the heel;
3. During this deceleration phase, the absorbed energy must be dissipated by displacement; and
4. The elastomer has to recover at a rate slow enough not to exert a significant upward pressure when, to use footwear as an example, the foot is on the ground, thus eliminating reverberations, but fast enough to regain its shape before the next step.
Moreover, such elastomers, and those formed in accordance with the present invention, must function effectively when subjected to multidirectional forces. They must be light in weight, be dimensionally stable at ambient temperature and have a very low compression set.
In order to achieve these properties, the polymer must be three dimensional, but must be very loosely bonded to achieve maximum distortion and acquire some of the physical properties of a liquid in order to achieve the low magnitude of compression set required.
Such a material has not been satisfactorily provided by the prior art. Polyethylene foams used as insoles in running shoes, for example, typically have a compression set measured according to ASTM standard test 395, method B, of about 25 to 30%. However, after three or four days of use, such foams are typically permanently compressed by an amount equal to nearly 70% of their original thickness, with an attendant decrease in shock absorbing capacity. The importance of a relatively low compression set to such shock absorbing materials is thus graphically illustrated.
As above noted, it is also imperative that shock absorbing materials for use in applications such as footwear be light in weight so as not to impede unduly the normal walking or running function. The density of such materials should be less than one gram per cc and greater than about 0.4 gram per cc, preferably from about 0.6 to about 0.7 grams per cc.
Properties such as those outlined above in conjunction with shock absorbing materials for footwear are also of significant value in absorption of industrial impact, for use as bumper stops, vibrational mounts, etc. Moreover, such a material would have substantial application as an acoustic damping material, particularly at low frequencies, e.g., below 500 Hz, and also at very high frequencies, e.g., above 10 KHz. Such acoustic damping properties are particularly significant in so-called constrained layer dampers, wherein a mobile center damping material is sandwiched between stiff outer surface materials.